Data stored in semiconductor memory cells may easily be read and written over, and also copied from one semiconductor memory device to another unless security features are implemented to restrict access to the memory cells.
Attempts have been made to provide security for data stored in semiconductor memory cells which are fabricated on a semiconductor chip. For instance, a microprocessor has been placed in the data path between a memory array of memory cells and an input/output (I/O) port on the chip. The stored data is secured by permitting access to the data stored in the memory array only if the microprocessor is initially provided with a recognized security code. This security technique, however, may be subverted because integrated semiconductor chips are finite state devices that may be reverse-engineered. Thus, a database attacker may be able to bypass the I/O port and directly access the memory array.
In some memory cell applications, such as integrated circuit cards or chip cards that include semiconductor memory cells, it is essential to assure that the data stored in the memory cells remains absolutely secure. This is especially important for applications where a database is utilized having no central database verification system. Another application concerns the use of memory cells in computer linked or on-line payment systems. The unauthorized modification of data stored in a chip card and the unauthorized copying of the data to another chip card for counterfeiting purposes, however, is not easily detectable using present technology. For example, the presently existing data security technique of providing holograms on chip cards may be compromised with relative ease and little probability for detection.
A need, therefore, exists for an improved technique for securing data stored in memory cells of a semiconductor memory device.